1. Field of the Invention
The present invention relates to flexible copper-applied substrates having excellent dimensional stability and adhesive properties.
2. Description of the Prior Art
Flexible copper-applied substrates are etched to form circuits, and various electronic parts are mounted on the circuits. Afterward, the circuits are incorporated in a system. In recent years, the mounting density of the parts on this type of circuit substrates is increased more and more, and the mounting procedure is also apt to be automated. The mounting is made under high-temperature conditions, e.g., by the use of automatic soldering, and therefore the substrates are required to have heat resistance. In addition, in the process of forming the circuits, the substrates suffer chemical and thermal changes such as etching and washing, and so it is necessary that they maintain their original plane form without curling and without peeling of copper from films.
In most cases, conventional polyimide flexible print substrates comprise polyimide films and copper foils applied thereon through an adhesive. However, with regard to this type of substrates, their heat resistance and electrical characteristics change with time inconveniently because of using the adhesive, and for this reason, these substrates can scarcely be applied to high-density circuits and automatic mounting systems. In order to solve these problems, some techniques have been suggested which are a method for thermally fusing polyimide films on copper foils (Japanese Patent Laid-open No. 181857/1982) and a method which comprises coating copper foils with a polyimide precursor solution, and then heating them to form polyimide films thereon (Japanese Patent Laid-open No. 111359/1986).
These improvement methods, since using no adhesive, can provide the substrates having good heat resistance and electrical characteristics, but the former method just described requires a thermal fusing step and other relative steps, and thus the same or more treatment steps as in the conventional case are necessary. Furthermore, there is the problem that when the polyimide films which have undergone the thermal fusing treatment at a high temperature are cooled to room temperature, the substrates tend to curl owing to a difference between thermal expansion coefficients of the resin and the copper foil. On the other hand, the latter method described above has no step of preparing any film, and the steps of this method can be simplified. However, since a high temperature is required in the polyimide formation step, the substrates are similarly apt to curl when cooled to room temperature. In addition, there is also the problem that adhesive strength between the polyimide film and the copper foil is not sufficient.
As discussed above, in the already suggested methods, the substrates tend to curl when the adhesive strength of the polyimide to the copper foil is heightened, and the adhesive strength deteriorates when the thermal expansion coefficient is controlled. In consequence, the known methods which have such problems are not sufficiently practical.